Multicolour printing systems that accumulate partial images on the final printed web of material require precise registration control. Partial images of the different primary colours need to be printed in registration with respect to each other as an additional requirement to the requirement of control of the position of the composite signature with respect to the web edges. This means that the control of the web has to be accurate over considerable distances. In paper post-processing technology, web guides are standard practice and common types generally operate based on active control units such as tilted frames as described, for example, in U.S. Pat. No. 3,411,683. Whereas such active steering systems are appropriate and very effective for adjusting the centreline of the web to a desired position, they are corrective control systems requiring an error to initiate a reaction. Therefore, the control units will continuously act and adjust the web position through adjustments of the tilted frame. An active control system will have a certain time constant for completing a correction. This time constant implies that the response of such a control system will have one or more fundamental frequencies, i.e. the position of the web will tend to vary cyclically. This means that the web is on average on the required centreline but limited amplitude drifts are inherently present in such systems and lead to low amplitude web walk or web meandering with frequency characteristics determined by the control system specifics. Moreover such systems are bulky and expensive and most effective at very high tension forces as common in rewinding applications. Therefore, there exists a need for a low cost passive alignment system for attachment to or integration in printer systems and other devices having a paper web in-feed.
Electrophotographic printing systems like the one described in U.S. Pat. No. 5,455,668 or an ink jet system of an architecture as described in U.S. Pat. No. 6,003,988 accumulate partial images over a distance along the web of over 1 meter and the temporal behaviour of the sideways web movement or “web-walk” as induced by an active steering system generally contribute to a significant extent to lateral registration errors between the colour separations. For high quality imaging it is generally desired that the contribution to lateral registration errors induced by web drift in between the transfer stations is better than 40 microns. This criterion is generally more severe than the requirement of registration with respect to the paper edges of the final printed pages.
High productivity roll fed printing systems benefit from large size unwinders that allow handling of large capacity print medium reels. Independent suppliers offer unwinder solutions and it is desirable to provide a flexible alignment solution to the printers that allows flexible connectivity to such unwinders without any cumbersome alignment procedure to align the different unwinders with the printing system. Hence, there exists a need for an alignment system that is insensitive with respect to mechanical misalignment of an external aligner, e.g. for a printing system.
A problem with thin paper webs is local Euler buckling. Euler buckling is buckling of a thin column into a bow-like or wave-like shape when placed under compression. The critical load which can be applied before buckling is initiated varies approximately as:
      P    cr    =                    π        2            ⁢      EI              L      2      wherein E is Young's modulus and I is the moment of inertia. For a quadratic cross-section the value of I is proportional to the cube of the thickness—hence the danger of mechanical buckling, edge curling or creasing of the paper web with a small thickness when a control device tries to push on one edge of the paper. Attempts to straighten a paper web will generally place one part of the web under tension and another under compression. Paper is not very extensible so movement has to be absorbed in compression. The most likely compression mode is buckling which results in edge folding or creasing within the bulk of the material. Another problem can occur if the alignment consists of many rollers over which the paper must pass. Due to the frictional forces between the paper and the rollers it becomes increasing difficult to align the web by sliding the paper sideways over the roller. This problem gets worse as the tension increases in the web.
A passive alignment system, which makes use of some of the recommendations given above and addresses a part of the desired features, is disclosed in U.S. Pat. No. 5,685,471. This known alignment system uses forcible guidance on one edge in an alignment zone where the paper tension is reduced with respect to the paper tension as exerted by the drive system of the printer. A slanted friction roller ensures the forced contact to the single aligning edge. It was found that such a solution is not appropriate in printing systems that need to address a broad range of printing media including high gloss substrates of high weight per unit area. Forcible alignment by such a slanted roller is especially questionable as a solution when the print media have a width exceeding 250 mm. Localised friction contacts are found to damage the medium surface by locally degrading the gloss, especially for higher medium weights that require higher forces for assuring forcible contact to the single side-guide.
There remains a need for an alignment system for continuous media with reduced webwalk and which is convenient and economical.